Screens for producing a printing plate for image reproduction

ABSTRACT

A screen for use in the production of a printing plate for image reproduction has individual mutually identical transparent surface elements surrounded by nontransparent surface portions, the centers of said transparent surface elements being situated on the crossings between lines in two coordinate line systems, the lines of which are spaced an equal distance apart and form an acute angle of 15*-60*, said surface elements having an at least substantial portion in the general form of a rhomb with its sides parallel to the lines of respectively one and the other coordinate line system, and the longest diagonal of the rhomb being at least equally long as said distance between the lines of the coordinate line systems.

United States atem [72] Inventor Anton Wilhelm Jemseby Allfarvagen 29, 791 47, Taby, Sweden [2]] Appl. No. 846,811 [22] Filed Aug. 1, 1969 [45] Patented Jan. 11, 1972 [54] SCREENS FOR PRODUCING A PRINTING PLATE FOR IMAGE REPRODUCTION 7 Claims, 4 Drawing Figs.

[52] US. Cl 96/116, 96/45, 96/363 [51] Int. Cl G03f 5/00 [50] Field of Search 96/116, 45, 36.3

[56] References Cited UNITED STATES PATENTS 59l,653 10/1897 Levy 96/116 7/l956 Wicklund ABSTRACT: A screen for use in the production of a printing plate for image reproduction has individual mutually identical transparent surface elements surrounded by nontransparent surface portions, the centers of said transparent surface elements being situated on the crossings between lines in two coordinate line systems, the lines of which are spaced an equal distance apart and form an acute angle of l560, said surface elements having an at least substantial portion in the general form of a rhomb with its sides parallel to the lines of respectively one and the other coordinate line system, and the longest diagonal of the rhomb being at least equally long as said distance between the lines ofthe coordinate line systems.

PATENTEDJI IHBTZ 3,634,092

F/G-Z F/G.4

INVENTOR ANTON WILHELM JEMSEBY BY Ham (0 mm ATTORNEY SCREENS FOR PRODUCING A PRINTING PLATE FOR IMAGE REPRODUCTION This invention relates to a screen for use in the production of a printing plate for image reproduction, comprising a plate or film with individual mutually identical transparent surface elements surrounded by nontransparent surface portions and uniformly distributed with their centers on the crossings between lines in two coordinate line systems, the lines of which are spaced an equal distance apart. The screen according to the present invention is essentially characterized in that each surface element has an at least substantial portion in the general form of a rhomb with its sides parallel to the lines of respectively one and the other coordinate line system, said lines forming an acute angle of from 15 to 60 with one another, and that the longest diagonal of the rhomb is at least equally long as the said distance between the lines of the coordinate line systems.

Compared to the known screens with square or circular transparent surface elements, the screen of the present invention offers several important advantages. In multicolor printing, the printing plates for the different color separation images, as is well known, are made with the screen in different angular positions. The known screens have but a few such angular positions in which a moire pattern is sufficiently suppressed, whereas the screen according to the invention can be adjusted to a greater number of such moire suppressing positions. In monochromatic as well as in multicolor work, the screen according to the invention gives less merging of the image elements in the dark areas of the image and fewer image elements per unit area in the brightest areas than the known screens mentioned above, with one and the same line density of the screens. In rubber stereo printing, the elongated image dots of the printing plate, which have been produced by means of the screen according to the invention, will be more stable, i.e., they are not so easily deformed and deflected by the pressure applied as when the known screens are used for producing the printing plate. In silk screen printing, the elongated image dots made with the screen according to the invention adhere better to the silk screen than the image dots made with the known screens. As will be seen in the following, some embodiments of the invention offer still further advantages.

The invention will be described in greater detail in the following with reference to the accompanying drawing in which:

FIG. 1 shows part of a screen made in accordance with the invention;

FIGS. 2-4 show three different modifications of the transparent surface elements in a screen according to the invention.

FIG. 1 shows part of a screen made in accordance with the invention and consisting of a plate or film which on one side has a surface pattern formed by means of nontransparent material and consisting of individual mutually identical transparent surface elements 1 free from the nontransparent material, and of surface areas 2 formed of said nontransparent material and surrounding the surface elements 1 which are uniformly distributed with their centers 3 on the crossings between lines 4 and 5 in two coordinate line systems, the lines of which are equally spaced apart. Each surface element 1 is in the form of a rhomb with its sides parallel to the lines 4 and 5 of respectively one and the other coordinate line system. The lines 4 and 5 form an acute angle A of 45 with one another so that the acute comers of the rhomb also have this angle. By suitable selection of the perpendicular distance between two opposing sides of the rhomb and the perpendicular distance between opposing sides of two rhombs following upon one another along the same line 4 or 5, each rhomb l besides is so dimensioned that its longest diagonal is at least as long as (and preferably much longer than) the distance between two successive lines 4 or 5.

Although the angle A in FIG. 1 is 45, it may be varied within the range l560. If the angle is greater than 60, the improvements provided by the invention in comparison with a conventional screen with square or circular transparent surface elements will not be so conspicuous, and it will be difficult to satisfy the requirement that the longest diagonal of the rhombs shall be at least as long as the mutual distance of the lines 4 and 5, respectively, of the coordinate line systems, without imparting to the screen an inappropriately large ratio between the total surface of the transparent surface elements 1 and the total surface of the nontransparent surface elements 2. If the angle A is smaller than 15, the surface elements will be too long and narrow for most purposes, and it will be difficult to avoid moire patterns and other objectionable pattern effects in the printed reproductions produced by the printing plates which have been made with the screen. For most uses of the screen, the best results are obtained with an angle A of between 30 and 45.

In the middle tones and the dark tones of images reproduced with the use of one or more screens according to the invention, where the distance between the lines 4 and 5, respectively, of the coordinate line systems is comparatively large, the eye can perceive lines which are free from printing ink and extend between the ink dots parallel to lines passing through the centers of the ink dots and corresponding to lines 4 and 5, respectively, in FIG. 1. In the middle tones and the dark tones of images reproduced by using one or more screens according to the invention which have rhombs with a very long and narrow form with an angle A of about l5-25, the eye besides can sometimes perceive diffuse pattern lines" resembling a moire pattern and extending within areas corresponding to the areas 6 in FIG. I, i.e., lines which in their lateral edge areas comprise opposed acute comers of the ap proximately rhombic ink dots.

To avoid such line effects in the reproduced images, or to deceive the eye, the rhombic form of the transparent surface elements of the screens are preferably modified in such a way that, although each surface element has a rhombic substantial portion, this substantial portion at least at one of the two acute corners of the rhomb merges into a secondary portion of the surface element, which secondary portion projects beyond the rhomb comer and the rhomb sides adjacent thereto. Although this secondary portion which merges with the substantial rhombic portion can be given many different shapes, it has proved advantageous to use a triangular or rhombic form, as is shown in FIGS. 2 and 3, 4.

The transparent screen surface element illustrated in FIG. 2 has a substantial portion 7 in the form of a rhomb whose acute corners which are overlapped by secondary portions 8, have an angle of for instance 30. Each secondary portion 8 is in the form of an isosceles triangle, the height of which lies in the extension of the longest diagonal of the rhomb 7 and whose base portion overlaps the respective acute rhomb corners and projects beyond the rhomb sides adjacent the acute rhomb corner. Although the top angle of the triangle 8 can by any suitable angle, it is often preferred to make it greater than the angle of the acute corners of the rhomb 7. In the present instance, the top angle of the triangle 8 is 45. The base of the triangle 8 can be greater than or preferably equal to the shortest diagonal of the rhomb 7, but in FIG. 2 it is shorter than the shortest diagonal of the rhomb. The triangles 8 may possibly be isosceles, and of different size.

The transparent screen surface element shown in FIG. 3 has a substantial portion 9 in the form of a rhomb whose acute comers which are overlapped by secondary portions 10, have an angle of e.g., 30. Each secondary portion 10 is in the form of a rhomb, the longest diagonal of which lies in the extension of the longest diagonal of the substantial rhombic portion 9, said rhomb overlapping said substantial portion 9 to preferably approximately half of its longest diagonal. The acute corners of the rhombic secondary portions 10 preferably have an angle greater than the angle of the acute comers of the substantial rhombic portion 10. In the present instance, the angle of the acute corners of the secondary portion 10 is 45. The shortest diagonal of the rhombic secondary portions 10 may be as long as or, preferably, shorter than the length of the shortest diagonal of the substantial portion 9. The two portions 10 may possibly be of different size.

FIG. 4 shows an embodiment where only one of the acute comers of the rhombic portion 11 is overlapped by a rhombic secondary portion 12 which in this instance by way of example has acute corners of the same angle as the acute corners of the portion 11. The rhombs 11 and 12 may be of the same or different size.

The screen according to the invention can be used in the same way as a conventional screen in a reproduction camera, and it can also be used in a per se known manner for contact printing of a transparent continuous-tone image on a light-sensitive layer, the continuous-tone image and the layer preferably being in contact with one another, whereas the screen is at some distance from the surface of contact between the transparent image and the light-sensitive layer, and exposure occurs through the screen and the transparent image by means of light from a single light source or preferably by means of intersecting light beams from several light sources placed in side-by-side relationship. At contact printing, the surface elements on the screen plate or film should preferably have an outwardly bulging outer surface to provide a lens action, whereby the image elements produced on the light-sensitive layer will have an advantageous distribution of the exposure with the maximum exposure in the center of each image element so that the image elements after development of the light-sensitive layer will have satisfactory etchability. As the tone values of the produced image can be controlled by etching, it is recommended for contact printing to use a negative transparent continuous-tone image so that the image produced on the light-sensitive layer is positive, whereby the etching results in a reduction of the image elements of the positive image and thus in a conversion of the image towards brighter tone values. In connection herewith, there is obtained a further advantage with modified screen surface elements according to e.g., FIGS. 2 and 3, viz that those portions of the image elements of the positive which correspond to the secondary portions 8 and are etched out before the substantial portion of the image elements corresponding to the substantial portions 7 and 9, respectively, so that adjustment of the etching time will not be as critical as if the screen surface elements were purely rhombic without secondary portions. The secondary portions of the image elements are etched out completely at the earliest in the brightest areas of the positive, and this is advantageous because it gives the desirable effect that the brightest areas of the positive seem to contain fewer image elements per unit area than the middle tones and the dark tones of the positive. It is pointed out that the complete etching out of the secondary portions of the image elements does not cause any appreciable pattern effects of the kind avoided by the provision of secondary portions for the transparent surface elements of the screen, since this complete etching out of the secondary portions of the image elements is effected only in the brightest areas of the positive where the image elements are very small and separated by comparatively wide image-free areas and therefore cannot produce the said pattern effects.

Although the image elements in the reproduced image, which are produced by the screen surface elements shown in the drawing, resemble the screen surface elements in shape, they are slightly rounded in relation thereto. In view hereof, the surface elements of the screen can be formed with slightly rounded comers, without detriment to the desired effect of the invention.

As previously mentioned, it is possible, when the screen according to the invention is used for the production of printing plates for the different color separations in multicolor work, to use the screen in a greater number of different angular positions for the different color separation printing plates than the screens which are presently used with square or circular transparent surface elements, without causing any objectionable moire patterns in the finished multicolor reproduction. The useful angular positions are determined by the angle A of the coordinate line systems 4, 5 and depend upon the presence or absence of secondary portions, for instance according to FIG. 2 or 3, of the transparent screen surface elements. The angular positions useful in each particular case can be readily established by experiments. In four-color work, it is usually found advantageous to use the screen in the angular positions 0, 45, and respectively, for the four printing plates. For the different printing plates for the color separation images of a multicolor image, it may often be suitable to use different screens according to the invention with difi'erent angles A for the lines of the coordinate line systems and/or with or without secondary portions, for instance according to FIGS. 2 and 3, of the transparent screen surface elements. It is also possible to use screens according to the invention in combination with conventional screens having square or circular transparent screen surface elements.

In the production of printing plates by means of screens according to the present invention, it is possible to produce intentionally certain types of moire pattern in the reproductions made with the printing plates, such moire pattern types being useful for printing of securities, for instance bank note or stamp printing, since it is practically impossible to counterfeit them unnoticeably by producing counterfeit printing plates by photographing the securities.

What I claim and desire to secure by Letters Patent is:

l. A screen for use in the production of a printing plate for image reproduction, comprising a transparent body sheet, a plurality of individual spaced-apart mutually identical transparent surface elements on said body sheet and opaque surface portions on said body sheet, surrounding said transparent surface elements and filling the spaces therebetween, said transparent surface elements being uniformly distributed with their centers on the crossings between lines in two coordinate line systems, the lines of which are spaced an equal distance apart and form an acute angle of from 15 to 60 with one another, each of said transparent surface elements comprising a primary element portion having the general form of a rhomb, and a secondary element portion at least at one of the two acute rhomb comers of said primary element portion and merging into said primary element portion, said secondary portion projecting from the acute rhomb corner and the rhomb sides forming that comer of said primary element portion, and each of said surface elements, formed by said primary and secondary element portions, having a longest dimension which is at least equally long as said distance between the lines of the coordinate line systems.

2. A screen according to claim 1, in which each of said surface elements has only one secondary element portion and said primary and secondary element portions have the same shape and size.

3. A screen for use in the production of a printing plate for image reproduction, comprising a transparent body sheet, a plurality of individual spaced-apart mutually identical transparent surface elements on said body sheet and opaque surface portions on said body sheet, surrounding said transparent surface elements and filling the spaces therebetween, said transparent surface elements being uniformly distributed with their centers on the crossings between lines in two coordinate line systems, the lines of which are spaced an equal distance apart and form an acute angle of from l5 to 60 with one another, each of said transparent surface elements comprising a primary element portion having the general form of a rhomb, and two secondary element portions at each one of the two acute rhomb corners of said primary element portion and merging into said primary element portion, said secondary element portions having the same shape and size and projecting from each one of the acute rhomb comers and the sides forming that corner of said primary element portion, and each of said surface elements, formed by said primary and secondary element portions, having a longest dimension which is at least as long as said distance between the lines of the coordinate line systems.

4. A screen according to claim 3, in which each of said secondary element portions is in the form of an isosceles triangle, the height of which lies in the extension of the longest diagonal of said rhombic primary element portion and the secondary element portions is in the form of a rhomb, the longest diagonal of which lies in the extension of the longest diagonal of said rhombic primary element portion.

7. A screen according to claim 6, in which the angle of the acute comers of said rhombic secondary element portions is greater than the angle of the acute corners of said rhombic primary element portion. 

2. A screen according to claim 1, in which each of said surface elements has only one secondary element portion and said primary and secondary element portions have the same shape and size.
 3. A screen for use in the production of a printing plate for image reproduction, comprising a transparent body sheet, a plurality of individual spaced-apart mutually identical transparent surface elements on said body sheet and opaque surface portions on said body sheet, surrounding said transparent surface elements and filling the spaces therebetween, said transparent surface elements being uniformly distributed with their centers on the crossings between lines in two coordinate line systems, the lines of which are spaced an equal distance apart and form an acute angle of from 15* to 60* with one another, each of said transparent surface elements comprising a primary element portion having the general form of a rhomb, and two secondary element portions at each one of the two acute rhomb corners of said primary element portion and merging into said primary element portion, said secondary element portions having the same shape and size and projecting from each one of the acute rhomb corners and the sides forming that corner of said primary element portion, and each of said surface elements, formed by said primary and secondary element portions, having a longest dimension which is at least as long as said distance between the lines of the coordinate line systems.
 4. A screen according to claim 3, in which each of said secondary element portions is in the form of an isosceles triangle, the height of which lies in the extension of the longest diagonal of said rhombic primary element portion and the base of which overlaps said primary element portion at the acute rhomb corner thereof and projects beyond the rhomb sides of said primary element portion adjacent said rhomb corner.
 5. A screen according to claim 4, in which the apex angle of said triangular secondary element portions is greater than the angle of the acute rhomb corners of said primary element portion.
 6. A screen according to claim 3, in which each of said secondary element portions is in the form of a rhomb, the longest diagonal of which lies in the extension of the longest diagonal of said rhombic primary element portion.
 7. A screen according to claim 6, in which the angle of the acute corners of said rhombic secondary element portions is greater than the angle of the acute corners of said rhombic primary element portion. 